Dropdown vs radio button
User have to choose type of food (from 3 or 4 choices) in our application. Default selection is used in 60 - 70% of cases. What will be better: dropdown or radio buttons?
impossible to say without a lot more information. Best option would be to design both then user test.
Keep in mind also the chances of this number of options changing. Requirements always change ;-)
Personally, I feel the default selection of a radio button works better than the default selection of a dropdown, because with the radio you see the other options.
It's impossible to give any meaningful rules on UI that would hold across all contexts. You might want to refer to your platform's UX guidelines for some guidance.
For the situation you describe, both widgets are probably reasonable choices. Radio buttons are useful if you want to emphasize the other available choices (e.g. to clarify the level of abstraction or precision), especially if the default choice is not very informative (e.g. “standard menu”). A drop-down list might be somewhat easier to operate with a keyboard, if a lot of data-entry is expected and users know the choices beforehand. It can also be adapted to allow users to enter something else (“combobox”). A big factor will be screen real estate, you can often save some space with a drop-down listbox compared to radio buttons.
If you really want to get to the bottom of this, you would have to test it with your own users but in most cases, it probably doesn't matter that much.